1. Field of the Invention
The invention relates to an evaporation system leak diagnostic apparatus that purges fuel vapor from a fuel tank to an intake system of an internal combustion engine via a canister.
2. Description of Related Art
One known evaporation system uses a canister to adsorb fuel vapor and then purges the fuel vapor adsorbed in the canister into an intake passage of an internal combustion engine at a suitable timing, in order to prevent fuel vapor produced in a fuel tank from being released into the atmosphere. If there is an abnormality such as a hole in the evaporation system, fuel vapor may leak out into the atmosphere.
Japanese Patent Application Publication No. 2004-308493 (JP-A-2004-308493), Japanese Patent Application Publication No. 2004-11561 (JP-A-2004-11561), and Japanese Patent Application Publication No. 2002-4958 (JP-A-2002-4958) describe evaporation system diagnostic apparatuses that diagnose a leak abnormality in the evaporation system in order to counter a fuel vapor leak abnormality. In JP-A-2004-308493, when diagnosing a leak, a closed system of only a fuel tank or a closed system of an entire evaporation system is formed, and the internal pressure of the fuel tank in a closed state is measured. It is determined that there is a leak abnormality in the fuel tank when the frequency with which the internal pressure measurement value is a value near atmospheric pressure is high, or the number of times that the internal pressure measurement value is a value near atmospheric pressure is large.
In JP-A-2004-11561, an electric pump is driven to pressurize the canister. Alternatively, intake negative pressure generated by operation of the internal combustion engine is introduced into the canister. Then the internal pressure of the canister is detected by a first pressure sensor, and a leak diagnostic is performed on the canister from the converged state of this pressure.
Moreover, after the leak diagnostic on the canister, the canister and the fuel tank are communicated, and the communicated space is pressurized by an electric pump provided on the canister side. Alternatively, intake negative pressure generated when the internal combustion engine is operated is introduced into the communicated space from the intake system via the canister. Then the internal pressure of the fuel tank is detected by a second pressure sensor, or the fuel tank internal pressure is introduced via a three-way valve and detected by the first pressure sensor, and a leak diagnostic is performed on the space (essentially a portion of the fuel tank) from the converged state of the pressure.
In JP-A-2004-11561, a leak diagnostic is performed on both the canister and the fuel tank using intake negative pressure generated when the internal combustion engine is operating and positive pressure that is actively created. In JP-A-2002-4958, intake negative pressure generated when the internal combustion engine is operating is introduced into only a canister, and a leak diagnostic is performed based on a change in the internal pressure of the canister. Moreover, the intake negative pressure generated when the internal combustion engine is operating is introduced into the fuel tank via the canister, and then the fuel tank is closed off from the canister, so as to form a closed space of only the fuel tank. A leak diagnostic is then performed based on a change in the internal pressure of the fuel tank in the closed off state.
In JP-A-2002-4958, the canister and the fuel tank are each placed in closed states, and intake negative pressure is introduced into each separately. Then a separate leak diagnostic is performed on each with a pressure sensor provided in each or with a single pressure sensor by using a three-way valve.
However, with the method described in JP-A-2004-308493, a leak diagnostic is performed only on the fuel tank or only on the combined space of the canister and the fuel tank, so a leak diagnostic distinguishing the fuel tank from the canister is unable to be performed.
In JP-A-2004-11561, a leak diagnostic is performed on the canister, and then a leak diagnostic is performed on the combined space of the canister and the fuel tank. However, with the method described in JP-A-2004-11561, if a pressure increase/decrease mechanism such as a pump is provided in the evaporation system in order to perform a leak diagnostic when the internal combustion engine is stopped, both the weight of the internal combustion engine and the cost increase. Moreover, two pressure sensors are used, which further increases both the weight of the internal combustion engine and the cost. In JP-A-2004-11561 an example is also described in which one pressure sensor is used, but the apparatus requires a separate three-way valve, which inevitably increases the overall complexity, makes the internal combustion engine heavier, and increases the cost.
In JP-A-2002-4958 as well, two pressure sensors are used, which makes the internal combustion engine heavier and increases the cost. Furthermore, in JP-A-2002-4958 as well, an example is described in which a three-way valve and one sensor are used in combination instead of the two pressure sensors, but again, this inevitably makes the internal combustion engine heavier and increases the cost.
JP-A-2004-11561 and JP-A-2002-4958 both describe methods that use intake negative pressure of the internal combustion engine. However, these methods are based on the assumption that the internal combustion engine is constantly operating, and a leak diagnostic is unable to be made when the internal combustion engine is stopped.